Duct sections for ventilating ducts are conventionally made with their four sides being continuous, each side having an end flange portion bent outward at a first 90.degree. bend and with at least a second 90.degree. bend back parallel to the duct surface. My co-pending application provides two additional flange portions; a spaced-apart third bend at 180.degree. to the second, and a fourth 180.degree. bend, (the narrow margin of material extending to the edge of the sheet) projected inward between the second and third flange portions. This construction presents no edge on which a workman might cut his hands; hence requires no special care to avoid injury.
Insertion, positioning and crimping of cornerpieces at the four corners at each end of a duct section, has required time and care. To manually position each of the right-angle cornerpiece arms, and hold it in place while crimping, and repeat this operation for each corner at each end of the duct section, slows the entire manufacturing process.